Mercury cell having rotating anodes

ABSTRACT

A NEW ROTARY ANODE, MERCURY CATHODE ELECTROLYTIC CELL FOR THE MANUFACTURE OF CHLORINE AND CAUSTIC ALKALIES FROM AQUEOUS ALKALI METAL CHLORIDE BRINES PROVIDES THE ADVANTAGES OF HIGH CURRENT DESNITY, HIGH CURRENT EFFICIENCY, LOW FLOOR SPACE AND LOW COST PER TON OF PRODUCT.

Dec. 26, 1972 R. M. COOPER 3,707,453

MERCURY CELL HAVING ROTATING ANODES Original Filed April 30, 1969 2 Sheets-Sheet 1 ROVM COOPER AGENT Doc. 26, 1912 R. M. COOPER 3,101,453

MERCURY CELL HAVING ROTATING ANODES Original Filed April 30, 1969 2 Sheets-Sheet z FIG-2 Y s k x x. & A

ROVMCOOPE/P AGENT United States Patent O US. Cl. 204-212 9 Claims ABSTRACT OF THE DISCLOSURE A new rotary anode, mercury cathode electrolytic cell for the manufacture of chlorine and caustic alkalies from aqueous alkali metal chloride brines provides the advantages of high current density, high current efiiciency, low floor space and low cost per ton of product.

This is a continuation of US. Ser. No. 820,459 filed Apr. 30, 1969, now US. 3,580,833.

This invention relates to novel structures suitable for use in electrolytic cells, particularly cells of the liquid cathode type and, more particularly, in mercury cathode electrolytic cells. The use of the structures of this invention in other cells of similar construction is also contemplated.

Horizontal mercury cells usually consist of an enclosed, elongated trough sloping slightly towards one end. The cathode is a flowing layer of mercury which is introduced at the higher end of the cell and flows by gravity along the bottom of the cell toward the lower end. The anodes are generally composed of rectangular blocks of graphite suspended from conductive lead-ins, for example, graphite or protected copper tubes or rods, in such a manner that the bottom of the graphite anode is spaced a short distance above the flowing mercury cathode. The bottom and sides of the trough are generally steel wtih a corrosion resistant hard rubber lining on the sides and under the cover. Concrete, stone or other non-conducting material may also be used for the sides. The lining may comprise concrete which is further coated with resin, or it may be natural stone set in a concrete lining.

More recently anodes of other materials than graphite have come into use. Titanium anodes, with one or more of the platinum group metals or their oxides coated on the side of the anode facing the cathode, are especially advantageous.

When the anodes are titanium coated with a platinum group metal, alloy or oxide thereof, the anodes are fabricated in any suitable form, for example, a sheet or expanded mesh, suitably reinforced as necessary.

In the operation of this type of cell, the electrolyte is an aqueous solution of any electrolyte which upon electrolytic decomposition will give the products desired. The electrolyte is introduced at the upper end of the cell and flows toward the lower end of the cell. Direct current passes through the solution between the anodes and the mercury cathode. When sodium chloride is the electrolyte, chlorine is formed at the anodes and passes to the top of the cell and out through an opening in the cell cover. Sodium is formed at the cathode as an amalgam with the mercury cathode. The sodium amalgam is withdrawn at the lower end of the cell, cycled to a decomposer packed with graphite where it is contacted with water to form sodium hydroxide, hydrogen and mercury. The mercury is recycled to the cell for reuse as the cathode. Brines of other electrolytes, particularly potassium chloride and lithium chloride, are also suitably electrolyzed in such cells.

In these cells, chlorine bubbles adhering to the anode r 3,707,453 Ice Patented Dec. 26, 1972 surfaces reduce the active surface in contact with the brine and contribute substantially to the total resistance in the cell. This increases the voltage required to force the electric current to flow through the cell especially at high current densities. Various anode designs are known in the art to facilitate the removal of bubbles and collection of the gas, including drill holes, channels and slots variously arranged. See, for example, US. Pats. 3,062,- 733; 3,174,923 and 3,268,427.

The mercury cell of the present invention is generally cylindrical and has a rotating anode. This structure materially improves the discharge of chlorine from the active face of the anode, improves the circulation of brine in the inter-electrode space and accelerates the flow of mercury and/or amalgam over the bottom of the cell. In addition, the anode is readily adjustable to maintain the interelectrode distance at optimum.

The mercury cathode cell of the present invention combines a cover and a bottom which are generally circular in shape and which are joined by sides generally cylindrical in shape, a mercury cathode disposed on said bottom and an anode generally circular in shape disposed between said bottom and said cover, inlet means for strong brine and mercury and outlet means for weak brine and for chlorine and amalgam products; a circumferential well around said bottom for amalgam accumulation and discharge; cathode current-carrying means attached to said bottom; rotatable axial anode support means attached perpendicularly to said anode and extending outside said cell; and anode current-carrying means, drive means and bearing means attached to said support means.

This invention, in another aspect, contemplates the improvement in the method of electrolysis of aqueous alkali metal chloride brines in a cell having a flowing mercury cathode and an anode parallel thereto and spaced therefrom, of maintaining the spacing between the anode and cathode at less than inch and rotating said anode about its axis. In a particularly advantageous form of the method of the invention, the anode is rotated to propel the brine relative to the anode at a weight rate of ilow such that its Reynolds number is at least 4000.

The anode support means of the cell of this invention is rotatable and is axially arranged with reference to the anode. The anode support means is suitably a shaft which may be tubular or solid. Advantageously, it is fabricated of titanium or alloys thereof which have superior resistance to the wet chlorine gas and chlorinated brine to which it is exposed. Use of titanium eliminates the necessity of providing sealing devices to prevent contact of these corrosive materials with the shaft. However, graphite or other metals, alloys or non-metals of satisfactory corrosion resistance and structural strength are suitable.

Suitable anodes for use in this invention are composed of cylindrical blocks of graphite. The lower surface facing the cathode is appropriately cut with concentric circular channels and advantageously a plurality of drill holes are provided connecting said channels with the upper surface of the anode. These channels and drill holes facilitate the escape of chlorine gas from the anode-cathode gap. Suitably rotary graphite anodes are up to about 12" thick when new and are used until they are only 1 or 2 inches thick when the danger of breakage is sufiicient to justify replacement. Larger anodes are suitably built up of segments of graphite.

Anodes of materials other than graphite are also suitable in these rotating anode mercury cathode cells, particularly titanium anodes having a thin coating over at least part of their surface of a platinum group metal or oxide thereof. The term, platinum group metal as used in this specification and claims means an element of the group consisting of ruthenium, rhodium, palladium,

osmium, iridium and platinum and alloys thereof. The term titanium includes alloys consisting essentially of titanium. These coated titanium anodes have the advantage of substantially complete resistance to corrosion and they therefore require little or no adjustment of the interelectrode spacing. For rotary anodes, they are particularly advantageous for their light weight which permits fabrication of large diameter anodes at low weight and low bearing loads. Suitably coated titanium anodes are fabricated of solid sheet or expanded mesh with reinforcing ribs or vanes to ensure presentation of a surface substantially parallel to the cathode surface.

The anode is suitably supported from above the cover of the cell or from below the cell. When support is from above, the anode current-carrying means are conveniently arranged on top of the cell cover. Rotary drive means, for example, gears or belts, and means for adjusting the elevation of the anode above the cathode are also suitably arranged above the cell cover. When support is from below the cell, several advantages appear:

(1) The cell cover and sides are entirely separate from the anodes and anode supports. The cover and sides are suitably constructed of very light weight materials easily handled by a mobile crane, for example, a cherry picker.

(2) Graphite anodes are not suspended from a post into the graphite and the anodes have no tendency to break loose and drop into the mercury cathode.

(3) Intercell bus bar costs are reduced since all buses are below the cell.

(4) Anode seal costs are eliminated since there are no anode seals.

(5) The cell is lighter and supporting structures are less expensive, especially using titanium anodes.

(6) Anode stub losses are radically reduced by supporting the anodes from below the cell.

(7) Electrical resistance is decreased because the current to the cell bottom and to the anodes is more uniformly distributed. Anodes supported from below provide better electrical contact between titanium and graphite.

In the preferred embodiment of the invention, the anode current-carrying means attached to the rotating axial anode support has rotatable and stationary elements, one of which has the form of an annulus and contains an electrically conductive liquid. The other element has at least one and preferably a plurality of dependent flanges partially imersed in the electrically conductive liquid. Preferably the stationary element contains the'liquid but alternatively the rotatable element contains the liquid. Suitably the conductive liquid is aqueous alkali metal chloride brine or liquid metal, e.g., gallium or liquid alloys. Preferably, however, the conductive liquid is mercury. This provides electrical contact with low voltage drop between moving metallic surfaces and requires no adjustment because there is no wear. Other means for conducting the current to the rotating anode are generally unsatisfactory. Brushes or other sliding contacts to the anode support means and anode wear rapidly, become dirty and severely pit the moving surface of the anode support shaft. The mercury filled contacting device avoids these problems and provides advantageous contacting means.

Generally, in this cell, as in most modern commercial cells, a pressure slightly below atmospheric is maintained in the cell. Air leakage inward is preferred to any leakage of chlorine out of the cell. For this reason, gas-tight bearings are not required in the cell of the present invention.

In operation, according to this invention, brine inlet and outlet flows, brine preparation and purification, amalgam flow to decomposers, recycle mercury flow, hydrogen and chlorine collection and treatment are conventional. Means for rotation of the anode are suitably hydraulic or electric motors or other conventional means of rotation. Speed of rotation of the anode is generally inversely related to its diameter, but the number of revolutions per minute is not adequate alone to define advantageous operating conditions. The size of the anode, its bottom configuration, the flow of brine and the interelectrode spacing are additional variable factors. All of these are taken into account in relating the operation to the Reynolds number which is a measure of the turbulence in the brine layer between the electrodes.

Reynolds number is defined and calculated in known manner as described, for example, in US Pat. 2,836,551. With rotating anode, the velocity varies radially from zero at the center to a maximum at the periphery. Reynolds numbers herein were calculated at the mid-point of the anode areas. Turbulent flow begins appreciably at Reynolds numbers of about 4000 and the voltage required to produce a given current density begins to decrease. In the range of Reynolds numbers of 10,000 to 20,000, the voltage decreases and apparently reaches a minimum at about 20,000. For operation at this minimum voltage, the anode in the cell of this invention is rotated at a rate to produce Reynolds numbers of at least 4000 and preferably in the range of 10,000 to 20,000. Since the Reynolds number is a function of the radius of the anode, among other factors, the r.p.m. of the anode is lower with anodes of larger radius and higher with anodes of smaller radius, other factors being constant. Operation in the advantageous range of Reynolds numbers permit lower voltages to produce higher anode current densities than in horizontal mercury cells with stationary anodes.

EXAMPLE I EXAMPLE II The cell was substantially of the design shown in FIG. 1, and the 9 inch diameter anode had concentric slots /s" wide by A" deep on /1" center lines and 7 vent holes drilled on 3" centers in each slot. Anode-cathode gap Was 0.1875". Rotation of the anode was varied from to 280 r.p.m. There was a 4% reduction in cell voltage at the same anode current densities as shown in Example I.

EXAMPLE III Using the cell of Example I having a solid 9 inch diameter rotating graphite anode and an interelectrode spacing of 0.1825 inch and maintaining an anode current density of 4.75 amperes per square inch, the rotational speed of the anode was varied to show the voltage reduction possible at increasing Reynolds numbers. The data obtained appear in Table 1.

TABLE 1 Reynolds Rpm. number Voltage The Reynolds numbers were calculated at the midpoint of the active anode area and represent an average over the entire anode area. The data show the advantageous reduction of voltage due to rotation of the anode while maintaining constant current density. Turbulent fiow of the electrolyte is shown at 76 r.p.m. and minimum voltage is achieved at abo t 300 rpm.

EXAMPLE iv The cell and rotating slotted anode of Example II was used to compare this cell with commercial horizontal mercury cells identified as E8 and Ell to show the advantage of the cell of this invention in terms of required floor space for given input of energy and therefore output of product. The data are shown in Table 2.

TABLE 2 Anode Amps] current Total ft. of density, Cell load, floor Cell amps/in. volts kiloamps area E8 3. 63 4. 32 33. 5 167 Rotating 4 3. 56 33. 5 234 Slotted 6 3. 75 33. 5 307 Anode 18 3g. 5 368 3 414 Cell 12 4.56 as. 5 459 13-11 4. 63 4. 16 100 295 Rotating 4 3. 56 100 295 Slotted 6 3. 75 100 405 Anode 18 100 500 0 100 581 Cell 12 4. 56 100 658 The data of Table 2 show that the rotating anode cell requires 30 to 50% less floor space than an E8 cell and operates with 7 to 16% lower cell voltage at the same total load. At higher cell loads, for example at the 100 kiloamperes used in the 13-11 cell, the rotating disc cell requires 41% less fioor space and 5 to 14% lower cell voltage.

FIG. 1 herewith shows one embodiment of the mercury cell of the invention in which a rotary graphite anode is supported above the cell cover. FIG. 2 shows a rotary titanium anode for use in a cell otherwise the same as the cell of FIG. 1. FIG. 3 shows an embodiment of the invention in which the rotary titanium anode is supported from below the cell.

More particularly, in FIG. 1, the cell generally consists of bottom 11, cover 12 and cylindrical sides 13. Mercury enters the cell at mercury inlet 14, covers bottom 11 and collects in circumferential amalgam well 15, flowing out of the cell via amalgam outlet 16. Brine flows into the cell via brine inlet 17 and out via brine outlet 18. Immersed in the brine is anode 19 having circular channels 20 in its lower face connected to the upper surface of circular anode 19 by drill holes 21. Bolts 22 hold anode 19 to shaft 23 which is supported by collar 24 on bearing surface 25. Lower bearing 26 is provided in cover 12. The cover is bolted to sides 13 by bolts 27 and a chlorine gas outlet is provided at 28. Shaft 23 carries drive means, suitably pulley 29 and rotating element 30 of the currentcarrying means. Stationary element 31 is attached to cover 12 and carries flexible anode bus 32 and block 33. Mercury 34 fills stationary element 31. Cathode current is supplied by flexible cathode bus 44 and block 45 attached to cell bottom 11.

FIG. 2 shows a rotary titanium anode for use in a cell otherwise the same as FIG. 1. Titanium sheet or expanded mesh 35 bearing a thin coating of platinum on the bottom side forms the active surface of the anode. Vanes 36 support the titanium from ring 37. These parts, including shaft 23 are all advantageously fabricated of titanium.

FIG. 3 shows a rotary titanium anode supported from below the cell. Corresponding parts bear the same numbers as in FIGS. 1 and 2. Shaft 38 rests in thrust bearing 39 suitably supported, for example, on concrete 40. Shaft 38 carries drive means 29 and rotating element 30 of the current-carrying means. Stationary element 31 carries anode bus 32 and block 33 resing on concrete pier 41.

Shaft 38 extends upward through the cell bottom 11 and is insulated therefrom by bearing 42 which also prevents outflow of mercury and brine. Mercury well 43 is formed by insulating bearing 42 and cell bottom 11 and mercury inlet 14 communicates with wall 43. The mercury flows across the surface of bottom 11 to amalgam well 15. Cathode current is supplied by flexible cathode bus 44 and block 45 attached to cell bottom 11.

What is claimed is:

1. A mercury cathode cell for electrolysis of aqueous alkali metal chloride brines, comprising the combination of a cover and a bottom which are generally circular in shape and which are joined by sides generally cylindrical in shape, a mercury cathode disposed on said bottom and an anode generally circular in shape disposed between said bottom and said cover, inlet means for strong brine and mercury and outlet means for weak brine and for chlorine and amalgam products; a circumferential well around said bottom for amalgam accumulation and discharge; cathode current-carrying means attached to said bottom; said anode having a lower surface facing said cathode; said lower surface being composed of titanium having a coating over at least part of said surface of a platinum group metal, alloy or oxide thereof; rotatable axial anode support means attached perpendicularly to said anode and extending outside said cell; and anode current-carrying means, drive means and bearing means attached to said support means including stationary and rotatable elements in contact with an electrically conductive liquid contained in one of said elements.

2. Mercury cell as claimed in claim 1 in which said conductive liquid is a liquid metal.

3. Mercury cell as claimed in claim 2 in which said liquid metal is mercury.

4. Mercury cell as claimed in claim 1 in which said rotatable element is attached to said anode support means and depends into said electrically conductive liquid contained in said stationary element.

5. Mercury cell as claimed in claim 1 in which said anode support means extends above said cover and aid stationary element is attached to said cover.

6. Mercury cell as claimed in claim 1 in which said anode support means extends below the bottom of said cell and sleeve means surround and insulate said support means from said brine and said mercury cathode.

7. Mercury cell as claimed in claim 1 in which said bottom has an inlet whereby mercury is supplied to form said cathode and an outlet whereby amalgam is removed from said cell.

8. Mercury cell as claimed in claim 1 in which said lower surface has the form of a solid sheet.

9. Mercury cell as claimed in claim 1 in which said lower surface has the form of expanded mesh.

References Cited UNITED STATES PATENTS 646,313 3/1900 Rhodin 2042l2 680,440 8/1901 Rhodin 2042l2 3,068,165 12/1962 Messner 204-212 X 3,409,533 11/1968 Murayama et a1. 204-219 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 

